In recent years, in response to strong social demands for low fuel consumption of vehicles, weight reduction of tires has been sought. Among tire members, weight reduction and the like of an inner liner also have begun. The inner liner is disposed inwardly in a tire, and has a function of reducing an amount of leakage of air (an amount of permeable air) from inside to outside of the pneumatic tire to improve air permeability resistance.
Currently, a rubber composition for such an inner liner employs, for example, a rubber blend mainly containing a butyl rubber. The rubber blend contains the butyl rubber by 70 mass % to 100 mass %, and a natural rubber by 30 mass % to 0 mass %. In this way, the tire is provided with improved air permeability resistance. In addition to butylene, the rubber blend mainly containing the butyl rubber contains isoprene by approximately 1 mass %, which acts with sulfur, vulcanization accelerator, and zinc white to achieve cross-linking between rubber molecules. In the case of normal blend, the above-described butyl-based rubber needs to have a thickness of 0.6 mm to 1.0 mm for a tire of a passenger car, and needs to have a thickness of approximately 1.0 mm to 2.0 mm for a tire of a truck/bus. In order to achieve weight reduction of such tires, use of polymer, which is more excellent in air permeability resistance than the butyl-based rubber and can provide an inner liner layer with a thinner thickness, has been requested.
Patent Literature 1 (Japanese Patent Laying-Open No. 7-215007) and Patent Literature 2 (Japanese Patent Laying-Open No. 11-254906) each propose a method for manufacturing a pneumatic tire including a core body having an outer circumferential surface resembling the inner surface shape of a tire when internal pressure is applied thereto in order to prevent uniformity of the tire from degrading. It is proposed that an unvulcanized tire is molded by sequentially applying an inner liner, a carcass ply, an unextended bead, a belt, a tread rubber, a side wall rubber, and the like to the outside of the core body. Then, the unvulcanized tire removed from the core body or the unvulcanized tire with the core body is input into a vulcanization press to be subjected to vulcanization, thereby improving uniformity of the tire.
In addition, in order to achieve weight reduction of a tire, it has been proposed to use a film made of a material including thermoplastic resin, instead of the above-described rubber composition. Thermoplastic resin has a smaller thickness than an inner liner of butyl rubber, achieving high air permeability resistance. A technique for winding an inner liner material obtained by extruding a strip of thermoplastic resin around a former has also been proposed. Here, when winding the strip, its portions overlap one another. A step difference will be generated at the overlapping portions, resulting in collections of air. The inner liner of thermoplastic resin having good air retainability cannot allow air in the collections of air to escape, with the result that a tire is manufactured while holding air even after vulcanization.
That is, air enters between the inner liner and the insulation or the carcass ply, thus resulting in a so-called “air-in phenomenon”, in which a large number of small balloon-like collections of air appear. Such spots resulting from air-in appearing in the tire inside surface give a user an impression of bad appearance. In addition, during traveling, the air causes cracks in the inner liner. Accordingly, the internal pressure is decreased, with the result that the tire may burst in the worst case.
Meanwhile, during traveling with the tire, large shear strain acts on a vicinity of a shoulder portion in the inner liner. When the material including the thermoplastic resin is used as the inner liner, this shear strain is likely to cause detachment at an adhesion interface between the inner liner and the carcass ply, with the result that air leakage takes place from the tire, disadvantageously.
In Patent Literature 3 (International Publication WO2008/029781), a tire is manufactured using a strip of film layer stack in which a thermoplastic resin and a thermoplastic elastomer are blended. With the film layer stack, gas barrier property and adhesive property can be improved, whereby bonding can be achieved between the ribbon-shaped strips. However, with this technique, a step difference is generated by overlapping the strip on a smooth outer circumferential surface of a former. The step difference may result in collections of air, and may generate air-in in the tire inner appearance after vulcanization. In addition, the strip is difficult to be adhered to the outer circumferential surface of the core body, particularly, the side surface from the bead portion to the buttress portion, and may peel off the core body, so that molding would not be performed.